performance_monitor_exception() is executed in a context with interrupt
disabled and preemption enabled. When there is a user space page fault
happened, do_page_fault() invoke in_atomic() to decide whether kernel
should handle such page fault. in_atomic() only check preempt_count.
So need call pagefault_disable() to disable preemption before reading
user stack.
Signed-off-by: Jiang Lu <lu.jiang@windriver.com> Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>